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タイトル: Seismometer Arrayによる地震波動の研究 : 第1報 1961年8月19日北美濃地震の余震について
その他のタイトル: Study of Earthquake Waves by a Seismometer Array : Part 1. Aftershocks of the Kitamino Earthquake of Aug. 19, 1961.
著者: 安芸 , 敬一
著者(別言語): Aki, Keiiti
発行日: 1962年11月15日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第40冊第2号, 1962.11.15, pp. 371-389
抄録: As reported in a preliminary paper by Miyamura and others, a multi-channel magnetic recording seismograph system was effectively used in locating small after- shocks of the Kita-Mino earthquake of Aug. 19, 1961. The seismograph system consists of a triparitite net of vertical component seismometers having the span of about 1 km and two horizontal component seismometers placed at one point. The seismic signals are sent to the recording station by cable, and recorded on a single 6 mm magnetic tape by the use of a delayed trigger mechanism. The records are reproduced on a paper by pen writing galvanometers with the paper speed of 120 mm per second. The amplitude and phase characteristics of the system (from the seismometer to the galvanometer) are shown in Fig. 1 and Fig. 2 respectively. The phase differences among the channels are reduced to less than 5 degrees for the frequency range from 0.2 to 20c/s. The accuracies of determination of the apparent velocity and direction of propagation are estimated as better than 4% and 3 degrees respectively for P waves from the aftershocks. In order to determine the epicentre and the focal depth of these aftershocks from the obtained apparent velocities and directions of propagation, we used a crustal model based on the refraction result by the Research Group for Explosion Seismology. This model has a horizontally homogeneous structure (Fig. 6). The average apparent velocity of P waves was 7.1 km/sec for the aftershocks (Fig. 5). This value combined with the average P-S time of 4.8 sec gives the average focal depth of 22 km for these shocks. The Earthquake Research Institute also operated other two temporary stations, which together with our station, make a large tripartite network covering the aftershock area. The P and S arrival times at these stations are used to check the validity of our estimation of the focal depth of the aftershock. It was found that the true depths of these shocks can be no deeper than about 12 km. The failure of correct depth estimation from the apparent velocities must be attributed to the improper choice of the crustal model. A modification of the model is proposed (Fig. 8), which gives better depth estimation and seems to be supported from other seismological and geological evidence We found for certain earthquakes that the apparent direction of the propagation of P waves must also be corrected for systematic error up to as much as 15 degrees. This correction can also be correlated with the detail of the local geology. Fig. 14 shows an example of these seismograms in which the initial P waves (P1) are the waves refracted horizontally along a possible high velocity channel and the later waves (P2) are the direct waves. It was emphasized that a careful examination of the systematic error is necessary in locating the hypocentre by the use of the apparent velocity and direction of propagation of P waves obtained at a single multi-seismometer station. Once the systematic error is corrected, however, this method of locating the hypocentre is very effective especially for very small earthquakes.
URI: http://hdl.handle.net/2261/12056
ISSN: 00408972


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